The present invention relates to a thin film semiconductor and method for making a thin film semiconductor, a solar cell, and a light emitting diode. More particularly, it relates to a thin film semiconductor and a method for forming a semiconductor film layer on a substrate having a plurality of porous layers defined therein having controlled and differing relative porosities.
As the material of the solar cell, various materials have been studied. Silicon, for which there are abundant reserves and which is free from apprehension of pollution, is the center of these efforts. Ninety percent or more of the amount of production of solar cells in the world are silicon solar cells as well. The tasks in solar cells are how to achieve a low cost, a high efficiency of conversion of light to electricity, a high reliability, and a small number of years for energy recovery. For the requests for high conversion efficiency and high reliability, single crystal silicon is most suitable, but it is difficult to fabricate single crystal silicon at a low cost. Therefore, at present, in the field of solar cells, particularly solar cells having a large surface area, active research and development is proceeding on solar cells using thin film polycrystalline silicon or thin film amorphous silicon.
In a thin film polycrystalline Si solar cell, the purity of the silicon is raised by refining techniques from metal class silicon using plasma or the like. An ingot is prepared by a casting process, and a wafer is prepared by a multi-wire or other high speed slicing technology. However, process for removing the boron and phosphorus from the metal class silicon, the preparation of an ingot of a good quality crystal by a casting process, enlargement of the surface area of the wafer, and a multi-wire or other high speed slicing technology require a very high grade of technology, so a substrate which is sufficiently cheap and has a good quality has not yet been fabricated at present. Further, the film thickness of the wafer is approximately 200 .mu.m, therefore a flexible substrate cannot be formed.
Amorphous silicon can be formed on the surface of a plastic substrate by a CVD (chemical vapor deposition) process. Therefore, it is possible to form flexible thin film amorphous silicon. As a result, solar cells having a wide range of applications can be formed. However, there are drawbacks in that the conversion efficiency is lower than that of the polycrystalline silicon and single crystal silicon, and the conversion efficiency deteriorates during use.
Single crystal silicon offers the promise of a high conversion efficiency and a high reliability. Thin film single crystal silicon can be fabricated by the SOI (Silicon On Insulator) technique, which is a manufacturing technique of integrated circuits etc., but the productivity is low. Using the SOI technique, the manufacturing cost becomes considerably high, this is a problem in application to a low cost solar cell. Further, the processing temperature in the preparation of single crystal silicon is relatively high, so it is difficult to form this on a plastic substrate or glass substrate having a low heat resistance. Since it is difficult to form single crystal silicon on a plastic substrate, the manufacture of flexible thin film single crystal silicon is difficult.
When constructing window glass equipped with solar cells, in other words, solar cells are arranged on a surface of a window glass, solar cars with solar cells arranged on the roof, etc., the use of a flexible solar cell is desirable from the viewpoint of the simplification of the manufacture and the ease of rational arrangement for enlarging the light receiving surface. Nevertheless, the only semiconductor silicon which can be used to make the flexible solar cells at the present time is amorphous silicon.